Unsolicited or other spam calls to telephones can be annoying to a subscriber who receives the calls. Unsolicited calls may be made from parties, organizations or campaigns with which the subscriber has no prior relationship. These calls can be made by robocallers even if such calls are prohibited by local telecommunication rules. In some cases, unwelcomed calls may be made to attempt to sell a product or otherwise solicit money from a subscriber. In more nefarious cases, an illegal attempt may be made to defraud the subscriber of money or property.
There are a few different general types of spam calls. Robocalls are machine generated calls that play announcements and may connect to live agents if the subscriber presses a digit. Push dialers are telemarketing campaigns that utilize predictive dialers. Live agents exist in some circumstances but are almost never used in large scale.
Robocalls are machine generated calls that typically just play a message. Sometimes there is an option provided to connect to a live agent. Robocalls often call from numbers with spoofed calling line identification (CLID) from an unreputable carrier or a hacked operator services trunk. Often random numbers are used within the North American Numbering Plan (NANP). Sometimes a fake national or international number is used. Sometimes an invalid number may be used. A large volume of calls may be sent to one NPA-NXX. The CLID is typically never the same on each call to avoid detection. These spam calls often do not leave a message on voice mail to avoid later detection. The spam calls often wait for a human answer, such as detecting the spoken word “hello” followed by silence. The spam call often can also detect standard voice-mail tones or non-silent periods that are longer than “hello”. Often robocalls have no live agent and just have a machine generating the call and playing the announcements. Sometimes there is an option provided to the subscriber recipient of the call to press a digit and speak to a live agent. There is often limited, if any, voice recognition made by the robocall. The robocalls often do not use voice mail message fingerprinting. Robocalls often use similar predictive dialing techniques to push dialers.
Spam push dialers are similar to robocalls in a number of ways. However, rather than being nearly entirely machine generated calls that typically leave a message, push dialers have machine generated calls that only connect to available agents once a human answer is detected. No live agent is connected until after a human answer is detected. Unlike most robocalls, push dialers may use voice mail message fingerprinting.
Current attempts to deal with spam blocking include do not call lists. However, nationally maintained telemarketing do not call lists only avoid cold calls from telemarketers that play by the rules. CLID blocking may be used, but only works on spam calls that block CLID. Caller reveal may also be employed, but only works on spam calls that block CLID or provide invalid CLID. Some telephone users may activate a Do Not Disturb (DND) mode, but it generally would only suppress calls during given periods or when requiring access codes. Using a DND system may be difficult to manage due to its complexity.
Other attempts to deal with spam calls include attempting to control incoming calls received on a network by requiring a challenge for the spam caller, but these systems have drawbacks. In some previous attempts, a challenge is sent to an incoming call, but no information obtained from the challenge is stored in the system. In such a system, the challenge process will need to be repeated each time a new call is received, which can be irritating for legitimate callers who will be subjected to repeated challenges each time they attempt to call.